1. Field of the Invention
The invention relates to a protective film of polarizer, polarizer and a liquid crystal device produced by use of the protective film of a polarizer.
2. Description of the Related Art
Applications of liquid crystal display devices are expanding year by year as power-saving and space-saving image display devices. Heretofore, one serious defect of liquid crystal display devices is that the viewing angle dependence of displayed image is large. Recently, however, various types of wide viewing angle-mode (for example VA-mode) liquid crystal display devices have been put into practical use, and in that situation, the demand for liquid crystal display devices is rapidly expanding even in the market such as the TV market which requires high image quality.
The basic constitution of a liquid crystal display device is such that a polarizer is provided on both sides of a liquid crystal cell. The polarizer plays a role in transmitting therethrough only the light polarized in a certain direction, and the performance of a liquid crystal display device is greatly influenced by the performance of the polarizer in the device. The polarizer is generally so designed that a transparent protective film is stuck to both the surface and the back of the polarizing element therein formed of an oriented, and iodide or dye-adsorbing polyvinyl alcohol film or the like. A cellulose acylate film, typically cellulose acetate, is highly transparent and readily secures good adhesiveness to polyvinyl alcohol for use as a polarizing element, and therefore it has been widely used as a protective film of polarizer.
With the recent expansion of applications of liquid crystal display devices, large-size and high-quality applications of TVs and others have become enlarging, and the demand for high-quality polarizers and protective films thereof is much increasing these days. In particular, use of large-size and high-quality liquid crystal display devices has become much more needed in various severe environments, for example, for outdoor use or the like than before. From such viewpoints, recently, for protective films of polarizers for use in liquid crystal display devices, improvement of both polarizer durability at high temperature and high humidity and polarizer durability at high temperature and low humidity has become greatly required.
Regarding this, a method of improving the polarizer durability at high temperature and high humidity has been investigated by making a polarizing element layer have an acidic pH at high temperature and high humidity. For example, Japanese Patent 4136054 discloses a protective film of polarizer that comprises cellulose acetate flakes containing an acid having an acid dissociation index in an aqueous solution of from 1.93 to 4.50.
On the other hand, WO2006/095815 discloses a method for producing a polarizer film having excellent wet heat durability and preferably, additionally having dry heat durability by processing, after stretching treatment thereof, a polyvinyl resin film containing iodine, iodide, crosslinking agent and/or water-proofing agent, with an acid processing solution having a pH to fall in a range of 2.4≦pH<6.0.